The invention relates to a process for producing cement clinker from raw meal which is preheated in at least one heat exchanger train traversed by the exhaust gas of a rotary tubular kiln, in particular a cyclone fluidized gas heat exchanger system, that has been precalcined, if necessary, in a precalcination stage and is burned in the sintering zone of the rotary tubular kiln into cement clinker, which is cooled in an afterxe2x80x94engaged cooler, in which process in the precalcination stage the exhaust gas flow of the rotary tubular kiln and an exhaust air stream of the clinker cooler are used separately or in common for the precalcination of the raw meal.
In installations for the production of cement clinker from cement raw meal, in order to avoid uneconomically long rotary tubular kilns and/or rotary tubular kilns of large diameter, and to keep the specific heat requirement of the cement clinker production process low, it is a known practice to engage upstream of the rotary tubular kiln, as seen on a material flow side, a precalcinator which is equipped with a second firing (besides the firing in the rotary tubular kiln). With a calcination degree of 90 to 95%, the specific heat consumption of the precalcinator alone already amounts to about 550 kcal/kg of cement clinker; i.e., the specific heat requirement of the precalcinator for the execution of the calcination reaction (neutralizing of the cement raw meal) lies far higher than the specific heat requirement for the after-engaged rotary tubular kiln. Thus, in modern cement clinker burning installations with a highly efficient precalcinator about 65% of the total fuel is burned up in the precalcinator alone.
In the development of rotary tubular kilns for cement clinker burning the tendency still is to use only as short as possible rotary tubular kilns, which, incidentally, are still supported on only two bearing stations. Short rotary tubular kilns, to be sure, have in consequence comparatively high kiln exhaust gas temperatures of, for example 1300xc2x0 C., which lead to a high thermal stress on the kiln intake chamber. The repair of a damaged kiln intake chamber would involve the shutting-off of the entire cement clinker production line. It would not be possible to cool the kiln intake chamber by means of a cooling jacket such as, for example a water mantle, because cement raw meals, that tend to agglutinate, would already cake on cooled kiln intake chamber walls already in the intake chamber of the rotary tubular kiln. This holds for all cement raw meals, but especially for raw meals that contain secondary components such as, for example, alkali compounds, chlorine, sulfur, heavy metals, etc.
Underlying the invention is the problem, especially in cement clinker production installations with a short rotary tubular kiln, to find a way in which the thermally highly stressed rotary kiln intake chamber can be protected and therewith the entire production installation can be driven operatively and reliably.
In the production of cement clinker according to the invention, a fine granular non-preheated material usable in the cement clinker production process, for example cold non-preheated cement raw meal, is introduced directly into the rotary kiln intake chamber and there it is suspended or dispersed in the rotary kiln exhaust gas, whereby the rotary kiln exhaust gas flowing into the intake chamber at a temperature of for example 1300xc2x0 C. is cooled abruptly to, for example, 1150xc2x0 C. and therewith the entire intake chamber is continuously cooled. By its introduction into the kiln intake chamber, the finely granular material is correspondingly heated abruptly and it is discharged with the exhaust gas out of the kiln intake chamber and added to the process raw meal, which then, after passing through a precalcination stage with fuel possibly present, passes into the rotary tubular kiln. The finely granular non-preheated material introduced into the rotary kiln intake chamber and dispersed there in the rotary kiln exhaust gas can simultaneously fulfill at least the three following functions. It acts as a:
a) Cooling medium for the rotary kiln intake chamber,
b) Catalyst for an NOx-reduction reaction for the purpose of reducing the NOx contained in the hot rotary kiln exhaust gas, which NOx comes essentially from the high-temperature combustion in the rotary tubular kiln,
c) Condensation nuclei on which gaseous or vaporous harmful substances such as in particular, for example, alkali chlorides and alkali sulfates can condense (freeze out or sublimate).
Especially if the cement raw meal contains secondary components such as, for example, alkali compounds, chlorine, sulfur, heavy metals etc. which evaporate in the range of the sintering zone of the rotary tubular kiln, for example as alkali chlorides and alkali sulfates compounds and condense on the non-preheated fine granular material introduced there, the dispersed fine granular material can be discharged at least partly from the kiln intake chamber via a separate partial gas outlet of its own.